membrane potential potential difference (voltage) across the cell membrane. in all cells of the...
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S3: Membrane Potentials and Action Potentials
Membrane potential Potential difference (voltage) across the
cell membrane. In all cells of the body (excitable and non-
excitable). Caused by ion concentration differences
between intracellular and extracellular fluid.
Membrane potential caused by diffusion of ions
4 mM 140 mM
142 mM
14 mM
Nernst potential
For each ion proportional to ratio of concentrations inside and outside the cell.
Always expressed as extracellular fluid has potential zero, and Nernst potential that from inside the cell.
± 61 log
Concentration insideConcentration outside(mV)
Nernst equation (37°C, for univalent ions):
Diffusion potential The membrane is permeable to several
different ions at the same time! Goldman equation:
Em = - 61 logCNaiPNa + CKiPK + CCliPCl
(mV)CNaoCNa + CKoPK + CCloPCl
(C) Concentration(P) Membrane permeability
Em =PK PNa PCl
Ptot Ptot Ptot
EeqK+ EeqNa+ EeqCl
Membrane permeability for K+ and Na+ (resting state) In resting nerve cells – open potassium
”leak” channels (“tandem pore domain”). 100x more permeable for K+ than Na+.
outside
Origin of Resting Membrane Potential
Contribution of Na+/K+ pump
Maintenance of concentration gradients for K+ and Na+ across cell membranes.
Electrogenic: creates additional negativity ~4 mV.
Measurement of membrane potential
Nerve Action Potential
Voltage-gated Na+ channels
1. Voltage gated K+ channels2. K+ leak channels3. Na+/K+ pump
Voltage-gated Na+ and K+ channels
Action Potential
Role of Ca2+
c(Cai)=10-7 mol/l
c(Cao)= 10-3 mol/l Strong concentration gradient (10 000-
fold concentration difference) In resting state, permeability for Ca2+
negligable. In heart cells, voltage-gated Ca2+
channels participate in action potential (plateau).
Action potential with plateau (heart)
Initiation of action potentials Action potentials will not discharge until
there is appropriate stimulus – depolarization. Exception – spontaneous rhythmicity.
Stimulus can be mechanical (mechanoreceptors), chemical (neurotransmitters) or electrical (heart muscle).
Positive feedback opens more and more Na+ channels.
Initiation of action potentials “Acute local potentials” must reach
threshold for eliciting AP “all or nothing” phenomenon.
Refractory Period Period of decreased excitability (relative r.p.)
or complete inexcitability (absolute r.p.)during and after action potential.
mV
Rhythmicity of Excitable Tissues Repeated spontaneous rhythmical
discharges (no outside stimulus). Heart (SA-node rhythmic activity),
intestinal smooth muscle (perystalsis) i CNS (breathing pace-maker).
Other excitable tissues can spontaneously discharge if threshold is lowered.
Spontaneous rhythmicity Resting membrane
potential -60 do -70 mV (close to threshold) activation Na + and Ca2+ channels.
Depolarizationa activates slow K+ channels repolarization i hyperpolarization.
Propagation of action potentials
Myelinated nerve fibersMyelin sheath: Insulation Decreases membrane
capacity every 1-2 mm along axon
myelin sheath is interrupted prekid mijelinske ovojnice Ranvier nodes 2-3 μm in length.
Saltatory conduction
Action potential are generated only in nodes of Ranvier energy saving and faster conduction (100 m/s).
Non-myelinated fibers conduction velocity 0,25 m/s.
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